Installing Pyserial in Blender
Answer:
I don't think there is a way to point a Python module installer to the Blender bundled Python.  So if you must use a third party Blender Python, there are generally 3 ways to go about that:


remove blender python sub-directory, blender will then fallback on the systems python and use that instead. Note that Python version must match the one that blender comes with.
Install a Python environment that's the same version as the bundled Blender Python to your system directory. (i.e. Python 3.3 64bit). Then install Pyserial (py3k) into that Python installation. Lastly copy the installed Pyserial module manually to the Blender Python installation under Blender\2.69\python. Installed Python modules can be usually found under the lib/site-package directory.
Most Python modules are just .py files that can be copies without going through a formal installation process. So, mostly likely, you can try to extract the module files from the installer, and just copy it straight to the Blender Python. This way, you don't have to keep a system Python just for installation purposes.


Non-answer: Do you really need to install it to the Blender Python? You can set up the PATH variable to basically load a module from any where on the disk.